userfaultfd: add user-mode only option to unprivileged_userfaultfd sysctl knob

With this change, when the knob is set to 0, it allows unprivileged users
to call userfaultfd, like when it is set to 1, but with the restriction
that page faults from only user-mode can be handled.  In this mode, an
unprivileged user (without SYS_CAP_PTRACE capability) must pass
UFFD_USER_MODE_ONLY to userfaultd or the API will fail with EPERM.

This enables administrators to reduce the likelihood that an attacker with
access to userfaultfd can delay faulting kernel code to widen timing
windows for other exploits.

The default value of this knob is changed to 0.  This is required for
correct functioning of pipe mutex.  However, this will fail postcopy live
migration, which will be unnoticeable to the VM guests.  To avoid this,
set 'vm.userfault = 1' in /sys/sysctl.conf.

The main reason this change is desirable as in the short term is that the
Android userland will behave as with the sysctl set to zero.  So without
this commit, any Linux binary using userfaultfd to manage its memory would
behave differently if run within the Android userland.  For more details,
refer to Andrea's reply [1].

[1] https://lore.kernel.org/lkml/[email protected]/

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Lokesh Gidra <[email protected]>
Reviewed-by: Andrea Arcangeli <[email protected]>
Cc: Kees Cook <[email protected]>
Cc: Jonathan Corbet <[email protected]>
Cc: Peter Xu <[email protected]>
Cc: Sebastian Andrzej Siewior <[email protected]>
Cc: Alexander Viro <[email protected]>
Cc: Stephen Smalley <[email protected]>
Cc: Eric Biggers <[email protected]>
Cc: Daniel Colascione <[email protected]>
Cc: "Joel Fernandes (Google)" <[email protected]>
Cc: Kalesh Singh <[email protected]>
Cc: Suren Baghdasaryan <[email protected]>
Cc: Jeff Vander Stoep <[email protected]>
Cc: <[email protected]>
Cc: Mike Rapoport <[email protected]>
Cc: Shaohua Li <[email protected]>
Cc: Jerome Glisse <[email protected]>
Cc: Mauro Carvalho Chehab <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: Mel Gorman <[email protected]>
Cc: Nitin Gupta <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Cc: Iurii Zaikin <[email protected]>
Cc: Luis Chamberlain <[email protected]>
Cc: Daniel Colascione <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

2 files changed, 18 insertions, 7 deletions

diff --git a/Documentation/admin-guide/sysctl/vm.rst b/Documentation/admin-guide/sysctl/vm.rst
index f455fa00c00f..d06a98b2a4e7 100644
--- a/Documentation/admin-guide/sysctl/vm.rst
+++ b/Documentation/admin-guide/sysctl/vm.rst
@@ -873,12 +873,17 @@ file-backed pages is less than the high watermark in a zone.
 unprivileged_userfaultfd
 ========================
 
-This flag controls whether unprivileged users can use the userfaultfd
-system calls.  Set this to 1 to allow unprivileged users to use the
-userfaultfd system calls, or set this to 0 to restrict userfaultfd to only
-privileged users (with SYS_CAP_PTRACE capability).
+This flag controls the mode in which unprivileged users can use the
+userfaultfd system calls. Set this to 0 to restrict unprivileged users
+to handle page faults in user mode only. In this case, users without
+SYS_CAP_PTRACE must pass UFFD_USER_MODE_ONLY in order for userfaultfd to
+succeed. Prohibiting use of userfaultfd for handling faults from kernel
+mode may make certain vulnerabilities more difficult to exploit.
 
-The default value is 1.
+Set this to 1 to allow unprivileged users to use the userfaultfd system
+calls without any restrictions.
+
+The default value is 0.
 
 
 user_reserve_kbytes
diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
index 605599fde015..894cc28142e7 100644
--- a/fs/userfaultfd.c
+++ b/fs/userfaultfd.c
@@ -28,7 +28,7 @@
 #include <linux/security.h>
 #include <linux/hugetlb.h>
 
-int sysctl_unprivileged_userfaultfd __read_mostly = 1;
+int sysctl_unprivileged_userfaultfd __read_mostly;
 
 static struct kmem_cache *userfaultfd_ctx_cachep __read_mostly;
 
@@ -1966,8 +1966,14 @@ SYSCALL_DEFINE1(userfaultfd, int, flags)
 	struct userfaultfd_ctx *ctx;
 	int fd;
 
-	if (!sysctl_unprivileged_userfaultfd && !capable(CAP_SYS_PTRACE))
+	if (!sysctl_unprivileged_userfaultfd &&
+	    (flags & UFFD_USER_MODE_ONLY) == 0 &&
+	    !capable(CAP_SYS_PTRACE)) {
+		printk_once(KERN_WARNING "uffd: Set unprivileged_userfaultfd "
+			"sysctl knob to 1 if kernel faults must be handled "
+			"without obtaining CAP_SYS_PTRACE capability\n");
 		return -EPERM;
+	}
 
 	BUG_ON(!current->mm);